How to carry out early defect detection

Experts at TÜV SÜD explain how early detection of critical assets can be carried out using an ‘electronic snooping’ sensor method.

Defects in materials of vessels and tubings are not uncommon. However, it is vital to detect them at an early stage to prevent unplanned downtime or even compromised safety and availability of process plants. Using the case study of a column, TÜV SÜD shows how this can be easily achieved with the help of acoustic emission testing (AT), while saving costs. 

Pressure vessels, tubing and other pressurised plant components must undergo periodic technical inspection, focusing on strength and tightness, but also possible damage mechanisms including cracking and corrosive attacks. In the majority of cases, discontinuities are not visible from the outside, so strength testing plus inside inspection have become common practice – a complex and costly approach, as it requires pressure vessels to be drained and cleaned before the test. This causes production loss due to the system shutdown and all that comes with it, from occupational health, safety measures and delays.
AT can replace internal visual testing and part of the external inspection of a pressure vessel during the periodic inspection scope if an appropriate plan is in place. It can also be used as a monitoring tool during pressure tests, also within the scope of occupational health and safety. If certain criteria are fulfilled, pressure equipment can be tested during operation without having to interrupt production. AT can also be used for permanent monitoring, possibly in combination with the online monitoring with UT or other NDTs. Depending on the mode of operation, the possibility of excluding some damage mechanisms and the on-site situation, draining and cleaning of the pressure vessel may be waived. A prerequisite for AT is stress application during measurement, which can be in form of pressure, temperature gradient or load. When applying pressure, the test pressure PTAET should be at least 1.1 times higher than the maximum operating pressure POP. 
Modern AT systems with fast processors and user-friendly operating software can process and display up to hundreds of localisations per second in real time. Also, the timing and speed of recording and analysis has increased a thousand-fold over the last few years. As a further advantage, 100% inspection and monitoring of a structure requires only a few sensors at fixed positions. Even large-scale vessels and plant components with complex geometries or difficult-to-access areas and installations can be inspected easily. Offering its real-time capability, AT is also suitable as a monitoring tool or occupational health and safety measure in gas-pressure testing. AT identifies an impending failure of the component under inspection at an early stage, allowing the test process to be safely interrupted or terminated at any time.  
AT is a non-destructive test method (NDT) for integrated detection and localisation of leakage and discontinuities. Application of the test pressure triggers changes such as plastic deformation or crack propagation in the material structure. Acoustic emissions (AE) are generated by sudden mechanical displacement in the material structure, which cause vibrations. The surrounding structure deforms elastically and bounces back, causing a transient elastic sound wave which propagates from its point of origin. To record the sound wave, the test engineers place piezoelectric sensors to the equipment under inspection before starting the loading. These sensors record the mechanical sound waves and convert them into electrical signals. The signals are transmitted to an analysis unit and to a test computer, where it is graphically displayed. Signal amplitude and the number of events are some of the parameters used for evaluation.
For evaluation, the signals are grouped into clusters based on local accumulations, where the number of signals detected in a single cluster indicates the degree of activity at that point. An effective method for assessing the measurement results involves categorising the signals and clusters in three classes according to their AE activity and intensity (Table 1), as a basis for planning the further process and pointing any necessary actions. 
Case study: Testing of a column    
A refinery operator commissioned TÜV SÜD to inspect a C4 column. The inside of the large vessel of the column was to undergo AT without inner visual examination. AT was to be carried out without process interruption. The vessel, manufactured in 2006 from fine-grain structural steel (P355 NH), had the following characteristics: height 74.3m, diameter 4.44m and volume 1,160m3. The wall thickness varied between 22 and 26 mm. Inside the column were numerous installed features, such as valve trays. 
Prior to the test, some measures were necessary, such as 88 piezoelectric sensors were distributed across the column (the column insulation was locally removed to enable them to be positioned). Inspection windows of 20 x 20 cm had to be opened at the sensors position. Using magnetic holders combined with a couplant, the sensors could be fixed directly to the metal surface of the pressure vessel, which ensured transmission of the sound waves during measurements to the devices.   
The test pressure (PTAET = 1.1 * POP) was applied by the plant manager’s control centre. The inspection was performed over 12 hours as real time online monitoring. In some areas, testing revealed active class 2 AE sources. TÜV SÜD recommended carrying out follow-up inspection of these zones in the form of classic weld inspection using UT phased array technique.

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